Different deficit patterns on word lists and short stories predict conversion to Alzheimer’s disease in patients with amnestic mild cognitive impairment

Episodic memory impairment is the most common and initial cognitive symptom of AD related to the early involvement of the medial temporal lobe (MTL). In this study, we compared performance on tasks routinely used in the neuropsychological assessment of episodic memory to evaluate which test is more sensitive in predicting subsequent progression to AD in patients with amnestic mild cognitive impairment (a-MCI). For this purpose, we conducted a longitudinal study in 61 patients diagnosed as a-MCI at baseline and followed for 3 years. Baseline memory performance on the word list and short story tests was analyzed to determine the diagnostic ability of the tests to predict subsequent conversion to AD. Results showed that stable a-MCI patients performed worse on word list than on story recall, whereas patients who later converted to AD tended to have similar poor memory performance on both tasks. Furthermore, a pronounced memory decay passing from immediate to delayed recall on the short story test was significantly associated with both higher risk and faster mean time of conversion to AD. We hypothesized that this pattern of results is a consequence of the early involvement in converter a-MCI of MTL areas which are fundamental in the consolidation of new memory traces.